Recently, kinds and the number of electric components of a vehicle have increased rapidly and exhaustion of a vehicle-mounted battery has become more intense. As a result, there is a desire to monitor the remaining capacity of the battery. In such a case, a method for attaching a current sensor to a battery post and detecting a degree of exhaustion of the battery by magnitude of this current is proposed. In that case, the periphery of the battery post has no space sufficient to attach the current sensor and also, a touch in the case of attaching the current sensor to the side of a harness connected to the battery post may damage the sensor in consideration of attachment of the harness itself, and measures taken against this include a method for mounting a current sensor on a relay member coupled through fastening means capable of connecting and disconnecting both of a battery terminal and a harness side terminal (for example, see PTL 1).